United States patent application US200710252654 describes designs for a frequency modulator based on a voltage controlled oscillator (VCO). A frequency modulator is provided for generating an output signal with a frequency that is a function of a modulation signal, wherein the modulation signal can assume different discrete modulation values, and a predetermined frequency value of the output signal is associated with each modulation value. The modulator contains a closed phase locked loop (PLL) with a loop filter for providing a first control voltage, with a voltage controlled oscillator for generating the output signal, and with a switchable frequency divider for deriving a frequency-divided signal. The divisor is the factor by which the input frequency of the divider is divided. Also a modulation unit is provided which gives, at a first output, values of the divisor that are a function of the modulation signal, and at a second output, a second control voltage that is a function of the modulation signal. The oscillator has a first control input connected to the loop filter and has a second control input connected to the second output of the modulation unit, and is designed to generate the output signal as a function of the first control voltage and the second control voltage. The frequency divider is connected to the first output of the modulation unit and is designed to derive the frequency-divided signal in such a manner that it has instantaneous frequencies that are a function of the divisor values.
A problem of the device known from US2007/0252654 is that a frequency step due to modulation does not accurately follow the modulation input.